Having used m3 since it came out I never experienced a hang. However, since I upgraded to m4 I frequently experience hangs when validation is taking place, typically right after I pull & update a major amount of changes from a repository.

When the hang happens, the CPU is at 104%. One page specifically always causes the hang (also on the systems of my co-workers who run m4), I removed it from the project and then validation succeeds but after the mentioned pull & update it often hangs again without a clear indication of what it is hanging on.

In my case there are typically quite a few more than that, but I know which one I need

To isolate the problem, I installed Eclipse 3.7.1 completely from scratch. Changed nothing and installed only Jboss tools 3.3m4, created a server for JBoss AS 6.10, created a dynamic web project with JBoss 6.x as target runtime. Added only the problematic file.

Got the hang nearly immediately. This time I tried OS X where the CPU is at a near constant 108.9% now, but I also tested this on Linux (Ubuntu 10.10 and 11.10), Max OS X 10.6 and Windows 7.